The present invention relates to a polyethylene film for packaging mainly light-sensitive materials, in particular, to a laminated film comprising linear low density polyethylene (hereinafter referred to as L-LDPE).
Polyethylene film has widely been employed as a packaging material, among which films employable have been polyethylene produced by a high pressure process (referred to "LDPE" hereinafter) and ones produced by a low/moderate pressure processes. Recently, L-LDPE a so-called third polyethylene having advantages of both high and low/moderated pressure polyethylenes has become commercially available. This L-LDPE attracts attention of those skilled in the art due to its low cost and high strength, which may meet the requirements for saving energy and resources.
However, L-LDPE has such a low transparency that as a general packaging film it may not be replaced for LDPE and has been considered difficult to be used owing to its poor processing properties even in a packaging film requiring no transparency. That is to say, L-LDPE which is a copolymer of ethylene with other .alpha.-olefin(s) requires not only an extra energy to form the film, but also installation of a new molding machine or replacement of existing molding machines.
Packaging materials completely intercepting light are used for packaging light-sensitive materials which will cause their quality and value to deteriorate on exposure to light. Such packaging materials must have excellent light-shielding properties; physical strengths such as breaking strength, tear strength, extent of impact perforation; Gelbo test strength etc.,; heat seal properties such as heat seal strength and hot seal strength; anti-static properties and the like. It is difficult to provide a single film material which possesses all the above-mentioned properties. Therefore, composite laminated films composed of a LDPE film comprising carbon black or pigments dispersed therein, and a flexible sheet such as paper, aluminum foil, cellophane etc. have heretofore been used in the art.
Such laminated films do not have sufficient physical properties and have disadvantages in that they are readily torn or pinholed during packaging work and in that heat sealed portions are readily separable. Furthermore incorporation of a great quantity of light intercepting material referred to "light-shielding material" hereinafter such as carbon black tends to reduce the physical strengths to a great extent. Thus the amount of the light-shielding material should be limited to about 3 weight % and the thickness of film should be not less than 70 .mu.m resulting in that the film becomes so bulky and rigid as to entail a poor processing properties on packaging and a high manufacturing cost.